The Saxon Wars were the campaigns and insurrections of the thirty-three years from 772, when Charlemagne first entered Saxony with the intent to conquer, to 804, when the last rebellion of tribesmen was defeated. In all, 18 campaigns were fought, primarily in what is now northern Germany. They resulted in the incorporation of Saxony into the Frankish realm and their forcible conversion from Germanic paganism to Christianity.
The Saxons were divided into four subgroups in four regions. Nearest to the ancient Frankish kingdom of Austrasia was Westphalia, and farthest was Eastphalia. In between the two kingdoms was that of Engria (or Engern), and north of the three, at the base of the Jutland peninsula, was Nordalbingia. Despite repeated setbacks, the Saxons resisted steadfastly, returning to raid Charlemagne's domains as soon as he turned his attention elsewhere. Their main leader, Widukind, was a resilient and resourceful opponent, but eventually was defeated and baptized (in 785).
First phase
In mid-January 772, the sacking and burning of the church of Deventer by a Saxon expedition -- precipitated by the missionary Albuinus' interruption of a worship service at the Saxon Marklo assembly and the blaspheming of their national gods -- was the casus belli for the first war waged by Charlemagne against the Saxons. It began with a Frankish invasion of Saxon territory and the subjugation of the Engrians and destruction of their sacred symbol Irminsul near Paderborn in 772 or 773 at Eresburg. Irminsul may have been a hollow tree trunk, presumably representing the pillar supporting the skies — similar to the Nordic tree Yggdrasil and apparently a common belief among the Germanic peoples. Charlemagne's campaign led all the way to the Weser River and destroyed several major Saxon strongholds. After negotiating with some Saxon nobles and obtaining hostages, Charlemagne turned his attention to his war against the Lombards in northern Italy; but Saxon free tenants, led by Widukind, continued to resist and raided Frankish lands in the Rhine region. Armed confrontations continued unabated for years.
Charlemagne's second campaign came in the year 775. Then he marched through Westphalia, conquering the fort of Sigiburg, and crossed Engria, where he defeated the Saxons again. Finally, in Eastphalia, he defeated them, and their leader Hessi converted to Christianity. He returned through Westphalia, leaving encampments at Sigiburg and Eresburg. All of Saxony except Nordalbingia was under his control, but the recalcitrant Saxons would not submit for long.
After warring in Italy, he returned very rapidly to Saxony (making it to Lippe before the Saxons knew he left Italy) for the third time in 776, when a rebellion destroyed his fortress at Eresburg. The Saxons were once again brought to heel, though Widukind fled to the Danes. Charlemagne built a new camp at Karlstadt. In 777, he called a national diet at Paderborn to integrate Saxony fully into the Frankish kingdom. Many Saxons were baptised. The Carmen de conversione Saxonum celebrates this event.
The chief purpose of the diet was to bring Saxony closer to Christianity. Missionaries, mainly Anglo-Saxons from England, were recruited to carry out this task. Charlemagne issued a number of decrees designed to break Saxon resistance and to inflict capital punishment on anyone observing heathen practices or disrespecting the king's peace. His severe and uncompromising position, which earned him the title "butcher of Saxons", caused his close adviser Alcuin of York, later abbot of Marmoutier Abbey, Tours, to urge leniency, as God's word should be spread not by the sword but by persuasion; but the wars continued.
In summer 779, Charlemagne again went into Saxony and conquered Eastphalia, Engria, and Westphalia. At a diet near Lippspringe, he divided the land into missionary districts and Frankish countships. He himself assisted in several mass baptisms (780). He then returned to Italy, and there was no Saxon revolt. From 780 to 782, the land had peace.